1. Field of the Invention
This invention relates to a speech decoding method and device for decoding an encoded signal produced by splitting the input speech signal in terms of a pre-set encoding unit, such as a block or a frame, and decoding the resulting encoded signal from one encoding unit to another.
2. Description of the Related Art
Up to now, a variety of encoding methods are known for encoding an audio signal (inclusive of speech and acoustic signals) for signal compression by exploiting statistic properties of the signals in the time domain and in the frequency domain and psychoacoustic characteristics of the human ear. As these encoding methods, a vector sum excited linear prediction (VSELP) encoding system, as a so-called code excited linear prediction (CELP) encoding system, or a pitch synchronous innovation-CELP (PSI-CELP) is recently attracting attention as low bit rate encoding system.
In the waveform encoding system, such as this CELP encoding system, an input speech signal is formed into a block or a frame, with a pre-set number of samples thereof as an encoding unit, and a closed-loop search for an optimum vector is executed on the block- or frame-based time-axis speech waveform by an analysis-by-synthesis method for vector quantizing the waveform for outputting an index of the optimum vector.
Meanwhile, in such waveform encoding system, such as the CELP encoding system, a cyclic redundancy check (CRC) code is appended to a crucial parameter. If an error is produced on CRC error check on the decoder side, parameters of the directly preceding block or frame are repeatedly used to prevent abrupt interruption of the reproduced speech. If the error is sustained, the gain is gradually lowered to establish a muted (silent) state.
However, if the parameters of the block or frame directly preceding error occurrence are used repeatedly, the pitch of the block- or frame-length period becomes audible, thus producing a strange perceptual feeling.
On the other hand, if the playback speed is retarded excessively by speed control, it is a frequent occurrence that the same frame be repeated or occur a number of times with a small shift. In such case, the pitch of the block- or frame-length period similarly becomes audible, thus again producing a strange perceptual feeling.